14C-labelled compounds

The incorporation of 14C into compounds at a suitable site often requires extensive and complicated syntheses, and thus a relatively long time. This usually means that 14C-labeled compounds are unsuitable for studies to be carried out during discovery. There are however, very rapid methods for incorporating 3 H into compounds. The newer methods, generally involving metal-catalyzed exchange reactions [15-18], in our experience, mean that suitable labels can often be prepared in 2 or 3 weeks. These time scales make the approach viable for discovery support. Additionally, and importantly, these methods can lead to specific incorporation of tritium. 

Crowley MP, Godin PJ, Inglis HS et al (1962) The biosynthesis of the pyrethrins . I. The incorporation of 14C-labelled compounds into the flowers of Chrysanthemum cinerariaefolium and the biosynthesis of chrysanthemum monocarboxylic acid. Biochim Biophys Acta 60 312-319... 

Fish. Initial reports to determine NP in fish referred to the non-specific analysis of 14C-labeled compounds administered in exposure experiments. Granmo and Kollberg [7] used a solubiliser employing pancreatic enzymes to extract 14C-labeled NP from exposed cod and analysed it by liquid scintillation counting. 

The effect of mineral and organic soil constituents on the mineralisation of LAS, AE, stearyl trimethylammonium chloride (STAC) and sodium stearate (main soap component) in soils was studied by Knaebel and co-workers [38]. The four 14C-labelled compounds were aseptically adsorbed to montmorillonite, kaolinite, illite, sand and humic acids and subsequently mixed with soil yielding surfactant concentrations of about 50 jig kg-1. The CO2 formation in the serum bottle respirometers was monitored over a period of 2 months indicating that the mineralisation extent was highest for LAS (49-75%). Somewhat lower amounts of produced CO2 were reported for AE and the stearate ranging from 34-58% and 29-47%, respectively. The mineralisation extent of the cationic surfactant did not exceed 21% (kaolinite) and achieved only 7% in the montmorillonite-modified soil. Associating the mineral type with the mineralisation kinetics showed that sand... 

The 14C-labelled compound was prepared from the 14C nitrobenzene that was obtained by converting the commercially available 4-nitroaniline-l-14C into the diazonium tetraflu-oroborate and reducing it with hypophospous acid (Scheme 3)25. 

In brief, the rats are anesthetized, followed by an injection of 0.2 mL of the test solution into the common carotid artery. The injection solution consists of a HEPES buffered Ringer s solution (containing 141 mM NaCl, 4 mM KC1, 2.8 mM CaCl2, and 10 mM HEPES, pH 7.4) which contains both the test substrate (e.g., a [3H]-labeled compound, about 10 /xCi) and a reference compound, which is highly extracted by the tissue (e.g., 0.1 /xCi [14C]n-butanol) in the presence or absence of transport inhibitors. If a [14C]-labeled compound is used as a test substrate, [3H]H20 can be selected as a reference compound. Rats are decapitated at 15 s after injection and the retina is removed. The retina is dissolved in 2 N NaOH and subsequently neutralized with 2 N HC1. The radioactivity is measured by liquid scintillation spectrometry. The RUI value, an index of the retinal distribution characteristics of the [3H] test substrate, is estimated using the following relationship ... 

The problem of radiochemical purity with respect to the chemical state of aged tritium or 14C-labeled compounds is still more acute. Because of the short range... 

Carbon-14 is usually produced in the form of barium carbonate which is therefore the starting point for the synthesis of 14C-labelled compounds fortunately, many such compounds at high specific activity are now available commercially. The isotope emits only /8-rays the maximum energy of which is about 0.15 MeV the rays are therefore only feebly penetrating and for this reason the radiation hazard in work with carbon-14 is small but special methods of assay are required to achieve great sensitivity. The most important of the assay procedures are ... 

The alkyne is formally derived by a 1,4-dehydrochlorination with ring opening, although subsequent work has shown that the cyclopropene ring-opens even at ambient temperature to 3-chloro-3-methylbut-l-yne and so a 1,2-dehydrochlorination of this is an alternative. The carbene could be formed by several routes from the cyclopropene, and an early experiment with a 14C-labelled compound indicated that the label at C-l of the cyclopropane became C-l of the carbene ... 

Preparation of the 14C-labelled compound 65 was accomplished in a manner analogous to Scheme 14. Friedel-Crafts acylation of 44 was conducted with [2-uC]-chloroacetyl chloride under aluminum trichloride catalysis to give the radiolabelled intermediate 64 (48 mCi/mmol). The carbonyl group of 64 was reduced with triethylsilane and the resulting alkyl chloride was reacted with piperazine 40 to provide 14C-labelled ziprasidone (65). The HCl salt of 65 was formed resulting in a final compound with a specific activity of 9.6 mCi/mmol. 

Since the doses are very small, conventional LC-MS techniques are sometimes not sensitive enough to assay samples from microdosing studies. Often accelerator mass spectrometry (AMS) and positron emission tomography (PET) are required for obtaining PK and distribution information, respectively. As described in Section 5.4, although AMS is capable of quantifying 14C-labeled compounds with attomole (10 18M) sensitivity, the technique is not useful for distinguishing between an NCE... 

Optical purity of barbituric acid enantiomers was determined by the isotope dilution method using 2-14C-labeled compounds 215 or by an NMR... 

Enander I, Sundwall A and Sorbo B (1962). Metabolic studies on A/-methylpyridinium-2-aldoxime - HI. Experiments with the 14C-labelled compound. Biochem Pharmacol, 11, 377-382. 

Problem 15.50 Compounds labeled at various positions by isotopes such as 14C (radioactive), D (deuterium), and 180 are used in studying reaction mechanisms. Suggest a possible synthesis of each of the labeled compounds below, using 14CH3OH as the source of 14C, D20 as the source of D, and H2I80 as the source of lsO. Once a 14C-labeled compound is made, it can be used in ensuring syntheses. Use any other unlabeled compounds. 

Robinson s original suggestion135 that the natural pyrrolizidine bases are derived in vivo from two molecules of ornithine (149) has been supported by studies using 14C-labeled compounds. Ornithine has been shown to be a specific precursor for the pyrrolizidine nucleus present in a variety of alkaloids. The feeding experiments carried out to date are listed in Table I. Three groups of workers have obtained labeled alkaloids after feeding 14C-labeled ornithines.136-138 Hydrolysis of the alkaloid to yield the free base, retronecine (127), has established that almost all the radioactivity is in the base portion. It is likely that ornithine is incorporated into retronecine after decarboxylation to putrescine (150) [Eq. (40)] since labeled putrescine is also specifically incorporated into retronecine (Table I). 

Compound 52 an effective anti-hypertensive drug, has been synthesized44b for biotransformation studies from p-chlorobenzoic acid-carbonyl-14C (53) in three steps modifying the methods of Sturm45, Hoefle46 and coworkers (equation 23). The intermediate 4-chloro-3-sulphamoylbenzoic acid-carbonyl-14C (54) with thionyl chloride yielded the acid chloride 55, which with 2-amino-3aa, 4a,5,6,7aa-hexahydro-4,7-methano-isoindoline gave the carbonyl-14C labelled compound 52. The UV spectra of 52 and of the unlabelled 52 as well as the Rf value on TLC of the single radioactive peak and of the fluorescent spot of unlabelled authentic specimen of 52 coincided. 

Radioactive [35S]-phosphorothioates have been used for in vivo evaluation of chronic or cumulative toxicity of oligonucleotides. To achieve site-specific 35S-labeling in phosphoramidite chemistry, either a mixture of [35S]/carbon disulfide/pyridine/triethylamine [329] or 35S-la-beled Beaucage reagent [330] have been used in the sulfurization step. 3H- and 14C-labeled compounds have been tools in investigations of the metabolic fate of the nucleobases of an oligomer [331]. 3H may be inserted at the 5 -end by formation of the 5 -carboxyaldehyde by Mofatt-Pfitzner reagent, followed by reduction with [3H]-sodium borohydride in 2-propanol... 

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